| Ionospheric scattering communication technology is highly favored in military communication and other fields due to its advantages of long communication distance,strong anti-interference ability,good security,and high reliability.Ionospheric scattering communication is a kind of over-the-horizon communication that utilizes the scattering effect of the ionosphere on VHF radio waves.Its communication channel is a multipath channel that varies with time,with selective fading in the time domain,frequency domain,and spatial domain,resulting in significant channel transmission losses.The channel characteristics different from conventional short range radio communication determine that the design of ionospheric scattering communication systems differs greatly from conventional radio communication systems in baseband signal processing.In this paper,based on the measured characteristics of the ionospheric scattering channel,the various modules of the ionospheric scattering communication system were analyzed and discussed in detail from the aspects of Matlab link simulation and baseband signal processing FPGA implementation.The main research contents and innovations are summarized as follows:(1)On the basis of explaining the characteristics of ionospheric scattering channels,a baseband signal processing scheme for ionospheric scattering communication systems was designed based on single carrier frequency domain equalization(SC-FDE)technology with good resistance to multipath fading.Aiming at the time-varying characteristics of channel,pilot based signal-to-noise ratio estimation method was explored,channel estimation algorithms based on the least squares criterion and MMSE equalization strategies were designed;combining diversity merging technology with SC-FDE technology to resist channel fading and improve system signal-to-noise ratio;using interleaving encoding to improve the performance of communication systems;in order to enhance the security of the system,a low complexity SM4 block cipher algorithm is used to encrypt the information and improve the anti eavesdropping ability.(2)In aspect of channel coding,LDPC encoding and decoding method that can meet the requirements of communication links has been implemented.On the basis of discussing the construction method of the parity check matrix,several commonly used LDPC coding algorithms and their computational complexity have been compared.The sum product algorithm,the minimum sum algorithm and the normalized minimum sum algorithm improved by the minimum sum algorithm have been emphatically studied.Matlab software was used to simulate the performance of LDPC code under different decoding algorithms and iterations in the ionospheric scattering channel environment,and the influence of different parameters on the system bit error rate was analyzed.At the same time,considering factors such as system performance and computational complexity,the normalized minimum sum algorithm was determined as the LDPC code decoding scheme in communication systems.(3)In terms of FPGA hardware implementation,the baseband signal processing scheme for the ionospheric scattering communication system proposed in this article is divided into two parts:the transmitter and the receiver.Each part is further divided into several modules.The design principle and implementation process of each module were discussed first,and then functional simulation was conducted on each module using Xilinx’s Vivado software.The correctness of each module’s implementation was verified through analysis of simulation results.Finally,board testing was conducted,and the actual system bit error rate was tested under different signal-to-noise ratio conditions.The hardware system’s actual performance and simulation results were in good agreement.The measured data shows that the designed baseband signal processing module of the ionospheric scattering communication system can meet practical application requirements and is of great significance in the field of long-distance special communication. |
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